I'm going to try to answer this question in two parts. The first part will be without discussing it with Ronn, and the second, after a discussion.
Part the First: Figure it out yourself!
How does the author/creator describe Dark Swashbuckling? It's on the front page of the Baenrahl site:
Some might consider this a ‘dark' setting. I will concede that it is dark, but it is not dark in a brooding or dismal way that you might expect from this type of setting. Sure, the characters are surrounded by a religious heirachy that seeks to crush them and their precious rebelion underfoot. But the players should be surrounded by sparks and flashes of the world they are trying to bring about. They are the heroes, and thus, must be allowed to be heroic.
So there's a lot of bad stuff going on, but the heroes can actually make a difference, and see the benefits of their efforts. Cool.
...the Baenrahl campaign is one that should be seen as befitting 'mature audiences.' ... the campaign should, at least occasionally, deal with subjects not often found in epic fantasy games. For example, the evil people in the campaign should do truly vile and unspeakable things. The player's should truely find them despicable. They should be glad when they take their final breath.
So the villains should not be cinematic, but rather really evil in a gut-wrenching, "I want them to die!" sort of way. Conversely, it could be surmised, the character's actions may be cinematic -- leaping into the fray, swinging from ornate light fixtures, etc. So the Dark is in the villains, and the Swashbuckling is in the heroes (or more precisely, in the action).
This makes sense. We want the action to be high-octane and cinema-worthy, where the players will not hesitate to jump from a moving sky-ship to the back of a winged mount to do battle. And we also want them to see the gritty underbelly of what the crushing weight of Urathear control has done to the world.
We may also want to plague them with 3-dimensional character plots rather than Three Musketeers "bed the wench and swing the sword" types of things. Actions should have consequences, and friends and loved ones can lead them into awful situations that can have lasting repercussions.
(Of course, most character plots generally die upon contact with the players -- they just don't care. But if I start penalizing them mechanically ... then they'll take notice.)
So, to sum up:
Dark Swashbuckling is a thematic technique where the villains are realistic in their motivations and vile in their deeds, but the action is high-octane and cinematic, and the heroes actually do make a difference.
Next up: A conversation with "The Man."